Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices. Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or “bubble jet”), uses electroresistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electroresistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No.5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission and micro fluid valving is needed which can be used with a broad range of liquid formulations. Apparatus are needed which combine the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by Matoba, et al in U.S. Pat. No. 5,684,519. The actuator is configured as a thin beam constructed of a single electroresistive material located in an ink chamber opposite an ink ejection nozzle. The beam buckles due to compressive thermo-mechanical forces when current is passed through the beam. The beam is pre-bent into a shape bowing towards the nozzle during fabrication so that the thermo-mechanical buckling always occurs in the direction of the pre-bending.
R. Tuli in U.S. Pat. No. 6,079,813 discloses an ink jet printhead device which uses a stressed thin film applied over a base substrate. Cavities are etched underneath the film creating a membrane film which has the tendency to bulge outward over cavity areas under the effect of internal compressed forces. The membrane film, and the bottom of the cavity, have electrodes deposited. An electric signal corresponding with input data is applied to two electrodes creating an electric field between electrodes. As a result, the membrane film is attracted and repelled against the fixed cavity bottom, following the electric signal and providing a variation of an adjacent ink chamber's volume ejecting an ink drop. In its displacement, the membrane film snaps, after passing the zone where the force created by the electric field adds to the internal compressed forces of the film, accelerating its displacement from one stable position into another.
A bistable, bilayer membrane actuator is used to open and close microvalves in a pumping device disclosed by Quenzer, et al. in U.S. Pat. No. 6,168,395. The membrane resides in a buckled configuration induced by compressive strains in the two different materials that compose the bilayer. Electrostatic forces are used to attract the membrane causing it to snap from a buckled-out to a buckled-in position, thereby opening and closing a valve. However, the electrostatic forces that can be reliably generated are weak and membrane sticking problems can limit the long term usefulness.
Park, et al., in U.S. Pat. No. 5,905,241 disclose a bilayer thin film microbeam actuator which snaps between stable states of buckle-out and buckle-in in response to mechanical load forces. The switch is used, for example, to trigger an airbag in response to over-threshold acceleration forces in a vehicle crash. The bilayer microbeam resides in a buckled position due to compressive strains introduced in the two materials of the beam during fabrication. In operation, an excessive acceleration of the mounting structure of the beam causes it to snap through to the opposite buckle state, opening or closing an electric switch.
Disclosures of a thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,087,638; 6,239,821 and 6,243,113. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,180,427; 6,254,793 and 6,274,056. The thermal actuators disclosed are of a bilayer cantilever type in which a thermal moment is generated between layers having substantially different coefficients of thermal expansion. Upon heating the cantilevered microbeam bends away from the layer having the higher coefficient of thermal expansion, deflecting the free end and causing liquid drop emission.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. Large and reliable force actuations can be realized by thermally cycling bilayer configurations. However, operation of thermal actuator style drop emitters, at high drop repetition frequencies, requires careful attention to the energy needed to cause drop ejection in order to avoid excessive heat build-up. The drop generation event relies on creating a large pressure impulse in the liquid at the nozzle. Configurations and designs that maximize the force impulse may therefore operate more efficiently and may be useable with fluids having higher viscosities and densities.
Binary fluid microvalve applications benefit from rapid transitions from open to closed states, thereby minimizing the time spent at intermediate pressures. A thermo-mechanical actuator with improved force strength and transition movement speed will allow more accurate and predictable microvalving and fluid metering.
Binary microswitch applications also benefit from rapid transitions from open to closed states, thereby minimizing the time spent at indeterminate electrical states. A thermo-mechanical actuator with improved force strength and transition movement speed will allow more accurate and predictable microswitching and electrical circuit control.
A useful design for thermo-mechanical actuators is a beam, or a plate, anchored at opposing edges to the device structure and capable of bowing outward at its center, providing mechanical actuation which is perpendicular to the nominal rest plane of the beam or plate. Such a configuration for the moveable member of a thermal actuator will be termed a deformable element herein and may have a variety of planar shapes and amount of perimeter anchoring. The deformation of the deformable element is caused by initially setting up thermal expansion effects within the plane of the deformable element. Both bulk expansion and contraction of the deformable element material, as well as gradients within the thickness of the deformable element, are useful in the design of thermo-mechanical actuators. Such expansion gradients may be caused by temperature gradients or by actual materials changes, layers, thru the deformable element. These bulk and gradient thermo-mechanical effects may be used together to design an actuator that operates by snap-through buckling maximizing the net magnitude and speed of mechanical actuation, thereby improving the performance of liquid drop emitters, fluid microvalves, and electrical microswitches.
Snap-through thermal actuators, which can be operated at acceptable peak temperatures while delivering large force magnitudes and accelerations, are needed in order to build systems that operate with a variety of fluids at high frequency and can be fabricated using MEMS fabrication methods.